An ultrasonic flow meter using a propagation time difference of an ultrasonic wave has been widely used in order to measure a flow rate of fluid flowing through a pipe.
The ultrasonic flow meter may be classified into a dry flow meter and a wet flow meter depending on a method of attaching an ultrasonic transducer. That is, the wet flow meter uses a method of boring a pipe, through which fluid flows, to insert an ultrasonic transducer into the pipe, whereas the dry flow meter uses a method of attaching an ultrasonic transducer onto an external wall of a pipe.
The wet ultrasonic flow meter is advantageous in that the flow meter directly contacts fluid to allow propagation only through the fluid, and uses a slope splitting method that splits a pipe along a radial direction of the pipe to facilitate measurement of a velocity of fluid flowing at boundaries of an internal wall of the pipe, and thereby, reducing a flow rate measurement error. The wet flow meter may be suitable for an off-the-shelf flow meter where an ultrasonic transducer may be pre-assembled in a pipe (a flow rate measurement pipe) prior to installation of the pipe, whereas the wet flow meter may not be fully suitable for a field construction where the wet flow meter needs to be coupled to an already existing water pipe or other fluid flowing pipes as a flow rate measurement pipe. Further, in case of the wet flow meter, since the ultrasonic transducer is inserted into the pipe, a flow rate measurement error may occur when a scale is formed, or foreign substance is attached to the vicinity of the transducer, as a result of which, the use of the dry ultrasonic flowmeter, which attaches an ultrasonic transducer onto an external wall of a pipe, tends to increase these days.
However, the existing dry ultrasonic flow meter has also been found to be problematic due to the difficulty of applying the slope splitting method to the situation where multiple pairs of ultrasonic transducers are provided to construct a multi-point ultrasonic flow meter. Typically a pair of ultrasonic transducers configuring a measurement line is arranged in an oblique relationship with respect to a flow direction of fluid. The dry ultrasonic flow meter has been found to be suitable for a method (a circumference splitting method) where a virtual straight line (a propagation path of an ultrasonic wave) connecting the pair of the ultrasonic transducers is set to pass through a center of a pipe. On the other hand, setting the straight line to pass through a boundary part of the pipe, instead of passing through the center of the pipe, has been found to be not easy. Accordingly, the dry ultrasonic flow meter has also revealed a problem in the measurement of a flow rate due to the limit in determining a flow rate at the boundary part of the pipe that significantly affects the flow rate of fluid.
In order to solve this problem, Korean Patent No. 10-1360363 (Title of Invention: Ultrasonic Sensor Unit Capable of Adjusting Incidence Angle and Dry Ultrasonic Flow Meter Using the Same) describes a wedge, of which at least part of a top surface is in an arc form, a mounting section that rotates along the top surface of the wedge to adjust an angle for generation of an ultrasonic wave, and an ultrasonic sensor mounted on the mounting section.
However, above-cited Korean Patent No. 10-1360363 has shortcoming that an ultrasonic wave cannot be easily transmitted and received since the top surface of the wedge is formed to have a certain curvature and the ultrasonic transducer cannot fixedly contact with the top surface of the wedge.